Extreme resistance in the past creating mess (7)
I believe the answer is:
Here is my best explanation:
'mess' is the definition.
(I have seen 'A mess' mean 'farrago' so perhaps 'mess' could also mean 'farrago')
'extreme resistance in the past' is the wordplay.
'extreme' becomes 'far' (I've seen this in other clues).
'resistance' becomes 'r' (abbreviation).
'in the past' becomes 'ago' ('ago' can be a synonym of 'in the past').
'creating' is the link.